In the field of optics it is very important that optical surfaces such as on lenses and mirrors conform to a required shape. For example a spherically concave mirror typically used in precision telescopes should have as little surface aberration as practical. Controlled grinding and finishing procedures are indispensable, but measurement for unacceptable aberrations is also necessary.
Techniques based on interferometry are used for detecting aberrations or mapping contours of a surface. These are based on combining optical waves so as to cause reinforcement or cancellation of optical power depending on phase differences between the waves. In a conventional application of the principles to measuring optical surfaces such as in Michelson interferometry, a beam is split into a reference beam and a second beam that is reflected from the test surface, so that the recombined beam has a standing wave pattern displayed as fringes representing the surface shape. Variations in the pattern are associated with surface aberrations. However, it is complicated to generate actual dimensional data from the fringes for the aberrations. Further, the optical system of conventional interferometers is complex and difficult to align.
The advent of lasers introduced coherent light and a corresponding capability for higher optical precision. As summarized in an article "Laser Diode Feedback Interferometer for Stabilization and Displacement Measurements" by T. Yoshino, M. Nara, S. Mnatzakanian, B. S. Lee and T. C. Strand, Applied Optics 26, 892 (1989), laser diodes are particularly useful because of stability, frequency tunability, low power, compact size and low cost. The article discloses an effect of self coupling such a laser to an external reflector so that the laser emission is altered by the reflection. A technique for incorporating detection of laser power into a feedback loop to lock in the phase angle of the emission, and display feedback current as a measure of linear displacement of the reflector, is also disclosed therein.